[big-green-man]

A search warrant has to specify exactly what is expected to be found and exactly where. If the police get a warrant to search your place because they believe you're manufacturing LSD and they find a dead body, that body cannot be used as evidence in prosecution.

So a drug sniffing dog alert being justification for a search, presumably they'd expect to find drugs upon opening the package. I think the same thing would apply here, they thought they found drugs, they didn't find drugs, so they can't do anything, let alone seize the property.